(1) Field of the Invention
The present invention relates to a diazo copying element. In particular, it relates to a one-component or binary type diazo copying element wherein a novel binder (binder resin) is used in a pre-coat layer overlying a substrate.
(2) Description of the Prior Art
The now generally utilized diazo copying elements comprise an original paper for use in diazo copying (high quality paper, medium quality paper or the like), a pre-coat layer overlying the surface of said original paper, and a diazo photosensitive layer overlying said pre-coat layer, wherein the diazo photosensitive layer constituting the one-component type diazo copying element contains a diazo compound (diazonium salt) but does not contain a coupling component, while the diazo photosensitive layer containing the binary type diazo copying element contains both a diazo compound and a coupling component. And, these diazo copying elements mostly include a back-coat layer formed on the back of the original paper (substrate).
The reason for forming a pre-coat layer in this instance is to aim at preventing a diazo photosensitive layer-forming liquid from permeating into the original paper as well as aim at smoothing the surface of the original paper for uniform coating of said forming liquid, increasing the image density and saturation, improving the image sharpness and the like.
The material (binder) for use in the above mentioned pre-coat layer normally includes vinyl acetate resin, vinyl acetate-styrene copolymer resin, acrylamide resin and the like.
However, the conventional diazo copying elements are defective in that since these resins used in the pre-coat layer act to promote the oxidation decomposition of azo dye (image area formed by coupling reaction between the diazo compound and coupler) caused by light and air, when the resulting copy is exposed to light and air its image area fades markedly with the results that the image density deteriorates, the image color tone changes and thus the copy becomes indistinct.